1. Field of the Invention
The present invention relates generally to printed circuit boards and more particularly to reducing thermally-induced mechanical damage of solder joints that connect electronic components to printed circuit boards.
2. Description of the Related Art
Thermally-induced mechanical damage of solder joints connecting electronic components to printed circuit boards (PCBs) has been a long-standing problem. The thermally-induced mechanical damage of solder joints is caused primarily by the different rate of expansion of a PCB and components connected to the PCB with temperature change. Virtually all substances expand when heated and contract when cooled. The magnitude of these changes in size for a particular material are described by a coefficient of thermal expansion (CTE) for that material. The larger the coefficient of thermal expansion, the greater the change in size for a given temperature change.
A PCB will typically have a CTE greater than that of connected electronic components. This is because most electronic components include silicon, and silicon has a relatively low CTE. When a component and a PCB are electrically connected using solder joints, these joints are subjected to stress during temperature change because the component and the PCB move relative to one another. This movement must be absorbed by the solderjoints. This phenomenon is depicted in FIG. 1A and 1B. FIG. 1A shows an electronic component 2, a PCB 3, and solder joints 1. The electronic component is arranged parallel to the PCB and is both separated from and connected to the PCB by solder joints 1. The solder joints are unstressed and are aligned perpendicular to both the PCB and the electronic component. FIG. 1B depicts the same component 2, PCB 3 and solder joints 1 after a temperature change. Both the PCB and the electronic component show thermal expansion, but the PCB has expanded to a greater extent than has the electronic component. As a result, some of the solder joints 1 are stressed and no longer are perpendicular to the PCB and the electronic component.
A second type of thermally-induced mechanical damage of solderjoints is due to warpage of electronic components caused by variations in the CTE of different materials which constitute the electronic component. During a temperature change, the magnitude of the warpage of the electronic component changes, which, in turn, varies the distance between portions of the electronic component and original plane of the printed circuit board. This change in distance subjects the solder joints connecting the electronic component to the rigid PCB to stress, i.e., tensile stress or compressive stress. This warping phenomenon is depicted in FIG. 2. FIG. 2 shows an electronic component 2, a PCB 3, and solder joints 1. The electronic component is warped, but is nevertheless arranged substantially parallel to the PCB. The electronic component is also both separated from and connected to the PCB by solder joints 1. The warpage of the electronic component causes the distance between the electronic component and the PCB to vary from a minimum distance h near an edge of the electronic component to a maximum distance h.sub.w near the center of the electronic component. Both the minimum distance and maximum distance may vary with temperature change, and, as a result, some of the solder joints 1 must conform to these variations and are subjected to stress.
Both these types of thermally-induced stress on a solder joint decrease the mean time between failures for the joint and can result in a complete failure of a PCB assembly. Hence, reducing thermally-induced stress is a key factor in mitigating thermally-induced mechanical damage of solderjoints connecting electronic components to PCBs and therefore improves the quality and reliability of the solder joints.
The problem of thermally-induced mechanical damage of solder joints has become more pronounced as the number of solder joints connecting an individual electronic component to a PCB have continued to increase. Larger numbers of solder joints for an individual electronic component have forced solder joints to become smaller and more densely packed into a given area of a PCB, while at the same time, the dimensions of the electronic components have increased to accommodate all the necessary solder connections. Additionally, the heat generated by the more complex components has become increasingly large. The combination of additional heat and larger component dimensions further aggravate the problem because small localized displacements accumulate over the entire component and result in larger net displacements.
In response to the problem of thermally-induced mechanical damage of solder joints, the PCB industry has tried numerous solutions, none of which has proved completely satisfactory. First, PCBs have been reinforced with cross-woven glass fibers. This has reduced the coefficient of thermal expansivity of the reinforced PCB when compared to an unreinforced PCB, but even the reinforced PCB has a higher coefficient of thermal expansivity than a typical electronic component. Second, various methods of cooling the heat generating components, including passive methods such as heat sinks and active methods such as cooling fans, have been tried. These methods are not practical in many applications due to cost and size factors. Lastly, some have tried to solve the thermally-induced stress problem by inserting a stress relief interface system, such as is described in U.S. Pat. No. 5,369,551, between the electronic component and the PCB. Such systems are not practical in many applications due to cost and size constraints.
From the foregoing it will be apparent that there is a need for a low cost, small and effective apparatus for mitigating thermally-induced mechanical damage of solder joints in order to increase the mean time between failures for the solder joints and improve reliability of PCB assemblies.